Reconcentrating spent alkali metal chloride solution for recycling toelectrolysis cell

ABSTRACT

IN THE ELECTROLYSIS OF AN ALKALI METAL CHLORIDE BY THE AMALGAM PROCESS WHEREIN A CONCENTRATED SOLUTION OF ALKALI METAL CHLORIDE IS SUBJECTED TO ELECTROLYSIS IN A CELL TO FORM CHLORINE AND ALKALI METAL AMALGAM, AND THE CONCENTRATION OF ALKALI METAL CHLORIDE IN SAID CELL IS REDUCED, THE IMPROVEMENT WHICH COMPRISES WITHDRAWING FROM SAID CELL THE ALKALI METAL CHLORIDE SOLUTION OF REDUCED CONCENTRATION, ADJUSTING ITS PH TO ABOUT 4 TO 10, EVAPORATING THE SOLUTION TO INCREASE ITS CONCENTRATION, ADDING THERETO MAKEUP FRESH SOLUTION AND RECYLCING THE ABLY EFFECTED BY MULTIPLE-STAGE FLASHING, THE CONCENTRATION ABLY EFFECT BY MULTI-STAGE FLASHING, THE CONCENTRATION OF SPENT SOLUTION RANGING FROM ABOUT 200 TO 280 GRAMS PER LITER AND RECYCLE SOLUTION HAVING A CONCENTRATION OF AT LEAST ABOUT 310 GRAMS PER LITER. THE PREFERRED ALKALI METAL CHLORIDE IS SODIUM CHLORIDE.

June 18, 1974 GUTH ETAL 3,817,847

RECONCENTRATING SPENT ALKALI METAL CHLORIDE SOLUTION FOR RECYCLING TO ELECTROLYSIS CELL Filed May 17, 1972 United States Patent US. Cl. 20499 4 Claims ABSTRACT OF THE DISCLOSURE In the electrolysis of an alkali metal chloride by the amalgam process wherein a concentrated solution of alkali metal chloride is subjected to electrolysis in a cell to form chlorine and alkali metal amalgam, and the concentration of alkali metal chloride in said cell is reduced, the improvement which comprises withdrawing from said cell the alkali metal chloride solution of reduced concentration, adjusting its pH to about 4 to 10, evaporating the solution to increase its concentration, adding thereto makeup fresh solution and recycling the concentrated solution to said cell. Evaporation is preferably effected by multiple-stage flashing, the concentration of spent solution ranging from about 200 to 280 grams per liter and recycle solution having a concentration of at least about 310 grams per liter. The preferred alkali metal chloride is sodium chloride.

This invention relates to a process for reconcentrating impoverished electrolysis brines by concentration through evaporation, preferably by flash evaporation.

In the electrolysis of alkali metal chlorides (the technique and details are described in Kirk-Othmer, Encyclopedia of Chemical Technology, 2nd ed., 1963, vol. 1, p. 668) by the mercury process, the alkali metal, for example sodium, is deposited on the mercury cathode where it forms sodium amalgam, while chlorine is evolved at the anode. Whereas in the electrolysis of an alkali metal chloride by the diaphragm process both solid salt and also brine can be used as starting materials, this is not possible in the mercury process. The difference is based on the fact that, in the diaphragm cell, the brine entering the anode compartment is guided through a diaphragm into the cathode compartment, resulting in the electrolytic formation of an approximately 11% NaOH solution which also contains all the unreacted sodium chloride. In the further course of the process, this solution, known in the art as the cell liquor," is worked up by evaporation into solid sodium chloride and a 50% sodium hydroxide solution.

In the mercury process, it is only that proportion of alkali metal chloride which is required for current conversion that is removed from the inflowing brine. The concentration of the alkali metal chloride in the infiowing brine is made as high as possible, in order to obtain as high a level of conductivity as possible and to prevent the hydrogen from being evolved, for example instead of the sodium, at the mercury cathode.

Since the change in concentration in the cell is kept low for the same reasons, efforts are made in practice, for example in the electrolysis of sodium chloride, to reach cell input concentrations of substantially 310 g./ liter of NaCl and output concentrations of substantially 28rd g./liter of NaCl. The dilute brine is saturated with solid salt and returned to the cells via a purification stage. An addition of brine would interfere with the water economy of the circuit and would make it impossible to reach the saturation concentration again. However, the measure described above is attended by the disadvantage that, for the electrolysis of alkali metal chlorides by the amalgam process, it is necessary either to use mined salt, or, where only brine is available, to set up a salination unit in front of the electrolysis plant.

It is also possible to introduce the impoverished brine itself instead of water into a salination plant although this involves difficulties on account of the large quantities of liquid to be circulated.

The present invention relates to a process for reconcentrating a dilute aqueous alkali metal chloride solution up to an alkali metal chloride content in the range of from 200 g./liter up to the saturation concentration for use in the electrolysis of alkali metal chlorides by the amalgam process, distinguished by the fact that the dilute, alkali metal chloride solution issuing from the cells of an alkali chloride electrolysis installation is adjusted to a pH value of from 4 to 10, after which this solution is subjected to a single-stage or multiple-stage evaporation, preferably flash evaporation, and the losses of the alkali metal chloride during electrolysis and of water during evaporation are compensated by the addition of brine with a concentration in the range of about 200 g./l. of alkali metal chloride up to the saturation concentration.

The advantage of the process according to the invention is that there is no need for the conventional form of brine purification in which all the brine in the system has to be passed through a purification stage. The quantity of brine to be purified, for example by precipitation and filtration, only amounts to substantially to to 20% of the original quantity because it is only necessary to treat that proportion of brine which corresponds to the output of chlorine and alkali liquor. In addition, there is no longer any need in the process according to the invention to dechlorinate the brine.

In principle, the process according to the invention is carried out by adjusting the brine issuing from the cells, at a temperature of from about 70 to 90 C., to the pH value of the brine entering the cells and then subjecting it to evaporation, preferably flash evaporation. Evapoartion of a corresponding quantity of water results in the formation of a brine which is substantially saturated with alkali metal chloride and which has a temperature of about 55 to C.

The reconcentrated brine can be delivered to the cells without any further purification. Purified and optionally preheated fresh brine is added before re-entry into the cell in order to compensate for the losses of brine which occur during electrolysis through the separation of chlorine and alkali metal.

In the process according to the invention, adjustment of the pH value of the brine to a value of from about 4 to 10 is preferably carried out before evaporation so that the brine can be returned to the cells immediately after it has been reconcentrated. Adjustment of the pH value can also be carried out at any other stage in the process should this be necessary under the particular process conditions.

Evaporation which takes place either in a single-stage or in several stages can be carried out in any way, although flash evaporation is preferably used for the purposes of the process according to the invention. Of particular use to the process according to the invention is the fact that considerable quantities of heat are generated at what are today normal current loads of from about 8 to 10 ka. per m3 of cathode surface, this being reflected in an increase in temperature of the brine between the cell inlet and the cell outlet. This increase in temperature can be used for evaporating the water and considerably reduces the amount of outside heat which has to be supplied for evaporation.

In principle, flash evaporation, which is a method known per se (cf. for example Ullmann, Encyklopadie der techn. Chemie, vol. 18, p. 463 (1967)), is carried out by exposing the brine coming from the cells to a reduced pressure in suitable vessels, for example flash vessels. Some of the water evaporates, so that there is an increase in the alkali chloride concentration of the brine. The pressures prevailing in the flash vessels during flash evaporation amount to between about 100 and 700 Torr.

One particularly advantageous embodiment of the process according to the invention is described in detail in the following with reference to the accompanying drawing which is a flow sheet of a multiple-stage flash evaporation system.

Referring now more particularly to the drawing, 1, 4, 8, 9 and 10 are pumps, 2 and 5 are flash chambers, 3 and 6 are condensers, 7 is a heat exchanger, 11 is a vacuum pump, 12 is an inlet pipe and 13 is an outlet pipe.

The brine at a temperature of 85 C., impoverished to a content of 280 g. of NaCl/liter, enters the flash chambers (2) through the pump (1). In the individual chambers which are under difierent. reduced pressures, from 100 to 700 Torr, the brine undergoes evaporation in stages, accompanied by cooling from 85 C. to 65 C. The vapors formed are deposited on the condensers (3). The salt solution cooled by expansion is then pumped by means of the pump (8) through the condensers (6) built into the flash chambers (5) where it is reheated to 105 C.

In a following steam-heated heat exchanger (7), the brine is then heated by external heat to such a high temperature (approximately 115 C.) that it is able to cool in stages during expansion to around 70 C. in the flash chambers (5). The condensate accumulating on the condensers (6) and (3) is collected and run off via the pumps (4) and (9) together with the fresh steam condensate. During expansion, water is evaporated in such a quantity that the required increase in concentration of from 280 to 315 g. of NaCl per liter is obtained. The concentrated brine is returned to the brine circuit via the pump (10). The vacuum pump (11) provides for the corresponding vacuum in the flash chambers. The cooling water for the condensers (3) enters via inlet pipe (12) and leaves via outlet pipe (13).

The process according to the invention is suitable for all kinds of installations used for the electrolysis of alkali metal chlorides by the amalgam process. Although it is preferably intended for application in the electrolysis of NaCl, it is by no means limited to this particular application.

The process according to the invention is illustrated in the following examples:

EXAMPLE 1 Conventional procedure An installation for the electrolysis of alkali metal chlorides by the mercury process with a daily output of around 5 5 0 tons of Cl: has a brine circuit with a throughput of substantially 1200 mfi/hr. The salt consumption amounts to substantially 950 tons of NaCl per day. The brine enters the cells with a temperature of around 70 to 75 C. and leaves them with a temperature of around 85 C. The brine issuing from the cells with a concentration of around 285 g./liter of NaCl is pumped via a dechlorination stage into a saturation stage and thence to a purification stage. The brine purification stage requires a filter surface of around 2000 m. and, for a residence time of 30 minutes, a container volume of around 600 cubic meters for flocculating the precipitate. A thickener cannot be used on account of the di- 4 mensions required for a brine circuit with a throughput of 1200 mfi/hr. Around 0.5 m3 of HCl (30%) and 1.0 m. of around 20% NaOH/hour are required for dechlorination and activation. Rock salt is used as the salt.

EXAMPLE 2 Introducing the alkali metal chloride from a brine extraction stage If the alkali metal chloride can be directly obtained in the form of crude brine, the following processes can be applied:

(a) salinating the brine after purification and using the solid salt for electrolysis; or

(b) the process according to the invention.

(a) Pre-purification of the salt through a salination unit The brine obtained from a pipeline passes through a purification stage and is introduced into a salination unit. The consumption of steam amounts to around 1 ton of steam at 4.5 atms. per ton of NaCl. Around 40 tons/hour of steam would be required for a daily output of around 950 tons of NaCl. The pure common salt crystallizate is used directly or resaturating the circuit brine.

During electrolysis, around 75% of the brine is returned to the cells without purification, 25% being subjected to purification. In this connection, it is important to remember that, on account of the small quantity of impurities, it is necessary to employ precoated filters so that filtration involves more outlay. The filter surface required amounts to about A of the filter surface of Example 1. All other particulars remain the same.

(b) Concentrating the impoverished brine by flash evaporation The brine issuing from the cells with a temperature of around C. is adjusted to the pH value of the brine entering the cells and delivered to the flash evaporation stage which, by evaporating around mF/hr. of water, provides brine which contains 315 g./l. of NaCl and which has a temperature of 70 C. The consumption of steam amounts to around 24 tons of steam at 2 atms. per hour. The concentrated brine is returned to the cells without further purification. Around 120 mi of purified brine per hour are introduced after preheating before entry into the cell. Only a small quantity of sodium hydroxide is necessary to restore the pH value of the brine to its original value after leaving the cell, otherwise no chemicals are required. The purification stage only has to handle around 120 mF/hr. of brine and its filter surface can be between 200 and 400 m. depending on whether or not it is preceded by a thickener.

It will be appreciated that the instant specification and examples are set forth by way of illustration and not limitation, and that various modifications and changes may be made without departing from the spirit and scope of the present invention.

What is claimed is:

1. In the electrolysis of an alkali metal chloride by the amalgam process wherein a concentrated solution of alkali metal chloride is subjected to electrolysis in a cell to form chlorine and alkali metal amalgam and the concentration of the alkali metal chloride in said cell is reduced, and whereafter the spent alkali metal chloride solution of reduced concentration is concentrated by flash evaporation, the improvement which comprises carrying the flash evaporation of the spent solution to about the saturation concentration of the alkali metal chloride, purifying substantially saturated fresh alkali metal cholride solution and adding it to the flash evaporated solution in an amount suflicient to compensate for the loss of alkali metal chloride during electrolysis and of water during evaporation, and forwarding the combined solution to the electrolysis cell.

2. The process of claim 1 wherein the alkali metal chloride is sodium chloride and the solution recycled to the cell has a concentration of at least about 310 grams per liter, the reduced concentration of the withdrawn solution ranging between about 200 and 280 grams per liter and its temperature ranging from about 70 to 90 C.

3. The process of claim 1 wherein evaporation is effected by multiple-stage flashing.

4. The process of claim 1 wherein the alkali metal chloride is sodium chloride, evaporation is efiected by multipie-stage flashing, the solution recycled to the cell has a concentration of at least about 310 grams per liter, the reduced concentration of the Withdrawn solution ranging between about 200 and 280 grams per liter and its temperature ranging from about 70 to 90 C.

References Cited GERALD L. KAPLAN, Primary Examiner D. R. VALENTINE, Assistant Examiner 

